1. Field of the Invention
The present invention relates to functional and in-circuit test systems and, more particularly, for providing backmatch impedance between the pin electronics driver and the device under test.
2. Statement of the Problem
Functional and in-circuit testing techniques are frequently utilized to test semiconductor circuits. Such testing systems utilize drivers that connect to transmission lines that in turn are connected to test points or nails in the device-under-test. The impedance between a driver and the test point becomes critical in accurately testing the device under test. Furthermore, such drivers must operate at high speeds such as 25 MHz. Hence, the backmatch impedance of the driver should match the impedance of the transmission line in order to control reflections and to provide an accurate representation of the signal at the device under test. Additionally, differences in the transmission line environment, especially due to fixturing difficulties, may cause impedance mismatching. Hence, a need exists to provide a plurality of different backmatch impedances between the driver and the device under test. Another reason for switching the backmatch resistance is to accommodate a common driver that is used for both in-circuit overdrive and functional testing.
One prior art approach to solving this problem is to provide a number of separate relays with one end of each relay connected to the output of the driver and the other end of each relay connected to a differing value of impedance. The impedances are then connected to the transmission line. Hence, by selectively activating the number of different relays, selected values of impedance can be switched into the transmission line between the output of the driver and the input to the device under test. While this functionally provides a solution to the above problem, the use of relays has several undesirable characteristics. First, such relays are generally expensive, occupy significant circuit board space, consume power which creates undesirable amounts of heat, and cannot switch at the high speeds desired for automated testing.
Hence, a need still exists to selectively provide the proper values of backmatch impedance between the driver and the device under the test in such a manner as to minimize space on the circuit board, reduce power consumption and thereby reduce heat generation, reduce the overall cost, and to operate at high operating speeds such as 25 MHz.
3. Solution to the Problem
The present invention provides a solution to the above problem by providing a plurality of separate driver circuits with each driver being connected to a different value of backmatch impedance. Each of the plurality of driver circuits have their individual backmatch impedances connected to the transmission line for delivering test signals to the device under test. The appropriate driver (and, therefore, the appropriate backmatch impedance) is selected by means of a solid state digital logic selection circuit which is capable of operating at speeds of 25 MHz or greater.
Hence, under the teachings of the present invention, the above problem is solved by selectively activating one of a plurality of available drivers having the desired amount of backmatch impedance. This arrangement is less costly to implement, occupies significantly less space on the circuit board, is capable of operating at high operating speeds, and consumes significantly less power so as to contribute less undesirable heat.